The present invention relates to a rubber composition for tire tread, which causes less heat build-up in the tire tread portion during running, enables reduced fuel consumption, remarkably improves braking power not only on a wet road surface but also on a snow-covered or frozen road surface, and which is especially excellent in driving property on a wet road surface.
In order to meet the recent social needs for material resource saving and energy saving, there has arisen a demand for reduced fuel consumption by automobiles. Thus, not only the development of car bodies themselves including engines which consume only a smaller amount of fuel has been made, but also studies of reduced fuel consumption tires which enable decreased energy loss have been increasingly made.
A rubber material having a smaller hysteresis loss has heretofore been sought as a raw material for a reduced fuel consumption tire. For example, the tread portion is said to be responsible for 50% or more of the hysteresis loss. Thus, natural rubber, polyisoprene rubber, polybutadiene rubber, styrenebutadiene copolymer rubber of a low glass transition temperature (Tg), and blends thereof, all of which have a small hysteresis loss, have been used as the rubber component of the tread portion. In an aspect of the rubber composition, there have heretofore been used rubber compositions containing a relatively small amount of carbon black having a relatively large particle size and as small an amount of a softener, such as an aromatic oil, as possible.
Recently, higher performance levels under adverse conditions have been sought for in a tire. For example, in the reduced fuel consumption tire developed as described above, a high braking performance capable of adapting the tire to a wet road surface and a snowcovered or frozen road surface from the viewpoint of safety without detriment to the reduced fuel consumption have been seriously demanded.
Where use is made of natural rubber, polyisoprene rubber, polybutadiene rubber, or low-Tg styrene-butadiene copolymer rubber, all having a small hysteresis loss as described above, the tire obtained has a defect in that the braking power, particularly on a wet road surface (wet skid resistance) is poor, leading to an extremely poor running stability. Where carbon black having a large particle size is used, deterioration of properties such as braking power on a wet road surface and abrasion resistance cannot be avoided although the fuel consumption can be remarkably reduced. Further, since the rubber composition used in the tread portion of a reduced fuel consumption tire contains a smaller amount of a softener blended therein, it is liable to harden at low temperature, with the result that the braking power on a snow-covered or frozen road surface (ice skid resistance) is not fully satisfactory.
On the other hand, high-vinyl polybutadiene and high-vinyl styrene-butadiene copolymer rubbers containing 50% or more of 1,2-vinyl bonds have recently been proposed as a material satisfying both reduced fuel consumption and enhanced wet skid resistance. However, since these rubbers have a high glass transition temperature (Tg), they are poor in abrasion resistance and are apt to harden at low temperatures, leading to notably poor braking power on a snow-covered or frozen road surface. Thus, they are also insufficient in satisfying all the above-mentioned properties.
As discussed above, there have been no proposals for tires which satisfy all the desired properties such as reduced fuel consumption, and enhanced braking power on wet roads and on a snow-covered or frozen road surface. Particularly in winter when the road surface becomes slippery as a result of snowfall or freezing, ordinary tires, including reduced fuel consumption tires, exhibit a very low braking power on the road surface as mentioned above. Thus, the use of a snow tire is often required. This forces a user to spend considerable time and labor in exchanging tires. In view of this, appearance of an all-season tire, which is essentially an ordinary summer season tire but which can satisfy the above-mentioned three desired properties, has been eagerly sought.